This invention generally relates to optical techniques for the measurement of the thickness of transparent articles. More specifically, this invention relates to the use of a plural component light beam for making such a measurement.
The use of optical techniques for measuring the thickness of transparent articles is known in the art. For example, U.S. Pat. No. 3,307,446 teaches the use of a slit type or linear illumination system in the measurement of the thickness of glass tubing. This system is relatively sensitive to the position of the article being measured and will suffer error due to the lack of parallelism of the two surfaces of the wall of the tubing being measured. This particular type of measurement error will be discussed in further detail herein. U.S. Pat. No. 3,807,870 teaches a somewhat similar system. Also known in the prior art is the use of interference patterns for making thickness measurements of transparent articles. Examples of these systems will be seen in U.S. Pat. No. 2,518,647; 3,709,610; 3,720,471; and 3,994,599. The present invention, by the use of a plural component beam of light as the illumination source, avoids errors due to non-parallelism of the surfaces being measured and the positional sensitivity problems of the prior art. Additionally, the complexity inherent in interference type measurement systems is likewise avoided.